Band-mixing and strain effects in InAs/GaAs quantum rings

We analyze for the first time the coupled influence of band mixing, strain, and piezoelectricity on electronic structure, eigenstates, and optical transition strengths for InAs/GaAs quantum-ring structures. It is shown that band mixing and strain alter the level energies and optical absorption coefficients significantly.     

Optical properties of a donor impurity in a two-dimensional quantum pseudodot

We investigate the linear and nonlinear optical properties of a donor impurity confined by a two-dimensional pseudoharmonic potential both including harmonic dot and antidot potentials in the presence of a strong magnetic field. Calculations are made by using the perturbation method and the compact density-matrix approach within the effective-mass approximation. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index changes have been examined. The results are presented as a function of the incident photon energy for the different values of the chemical potential of the electron gas and the zero point of the pseudoharmonic potential. The results show that the optical properties of a donor impurity in a two-dimensional pseudoharmonic QD are strongly affected by the zero point of the pseudoharmonic potential, the chemical potential of the electron gas and the Coulomb interaction.     

Effects of intersubband interaction on multisubband electron transport in single and double quantum wells

The multisubband electron transport properties are studied for doped single quantum well and gated double asymmetric quantum well structures. The effects due to intersubband interaction and screening of the ionized impurity scattering are also investigated. We show that intersubband coupling plays an essential role in describing the screening properties as well as the effect of ionized impurity scattering on the mobility in a doped single quantum well. For coupled double quantum well structures, negative transconductance is found theoretically which is due to resonant tunneling between the two quantum wells.     

Impurity-modulated Aharonov–Bohm oscillations and intraband optical absorption in quantum dot–ring nanostructures

In this work we study the electronic states in quantum dot–ring complex nanostructures with an on-center hydrogenic impurity. The influence of the impurity on Aharonov–Bohm energy spectra oscillations and intraband optical absorption is investigated. It is shown that in the presence of a hydrogenic donor impurity the Aharonov–Bohm oscillations in quantum dot–ring structures become highly tunable. Furthermore, the presence of the impurity drastically changes the intraband absorption spectra due to the strong controllability of the electron localization type.     

Optical properties of impurity-bound polaron in a parabolic quantum dot

The linear and nonlinear optical properties of an electron, which is bounded to a Coulomb impurity in a polar semiconductor quantum dot with parabolic confinement in both two and three dimensions, are studied by using the Landau-Pekar variational method and the compact density-matrix approach. With typical semiconducting GaAs-based materials, the linear, third-order nonlinear, total optical absorption coefficients and refractive indexes have been examined. We find that the all absorption spectra and refractive index changes are strongly affected by the electron-LO-phonon interaction. The results also indicate that the polaron effect increases with decreasing dimensionality of a quantum dot.     

Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

Effect of Coulomb correlations in a variable-valence impurity system and phonon drag on the thermoelectric constants in two-dimensional systems

The temperature behavior of the longitudinal Nernst-Ettingshausen coefficient in 2D systems is studied theoretically taking account of phonon drag and Coulomb correlations in a system of mixed-valence impurities at low temperatures. It is shown that the effect changes sign at the transition from entrainment to scattering by a correlated system of impurity centers. A sign change does not occur in the case of scattering by randomly distributed impurity centers. This temperature behavior of the Nernst-Ettingshausen coefficient is due to the radical rearrangement of the impurity system as a result of strong Coulomb correlations present in a system of impurities with mixed valence. As a result, the character of the scattering of charge carriers by the correlated system of charge centers changes substantially. Fiz. Tverd. Tela (St. Petersburg) 40, 553–556 (March 1998)     

Electron Raman scattering of a hydrogenic impurity in a disc-shaped quantum dot

We have carried out the theoretical calculation of the differential cross section for the electron Raman scattering process associated with a hydrogenic impurity in a disc-shaped quantum dot (QD). We consider the impurity states confined in a disc-shaped QD with parabolic potential in the presence of an external electric field. Effects of the electric field and the confinement strength on the differential cross section are investigated. We make a comparison about the Raman intensity between with and without the Coulomb interaction. We found that the differential cross section of the hydrogenic impurity in a disc-shaped QD dependent strongly on the confinement strength, the external electric field intensity and the Coulomb interaction.     

Long range absorption and other optical-model effects from strong inelastic coupling

An optical potential component is constructed to represent the effect of a strongly coupled inelastic excitation upon elastic scattering. In the particular case of quadrupole Coulomb excitation a long range imaginary potential component is derived in closed form. The effects of long range absorption upon the elastic scattering are considered in a general way by inserting this potential into a weak absorption model and deriving an elastic scattering cross section in closed form. Below the Coulomb barrier the formula takes a simple form which may be related to the semiclassical theory of Coulomb excitation. The potential component arising from nuclear excitation of an inelastic state may be evaluated numerically on a computer. Two examples computed (50 MeV α-scattering on ¹⁵⁴Sm and 60 MeV ¹⁶O scattering on ⁴⁰Ca) exhibit strong l-dependence in the potential component.     

Linear and nonlinear optical susceptibilities in a laterally coupled quantum-dot–quantum-ring system

Linear and nonlinear optical susceptibilities in a laterally coupled quantum-dot–quantum-ring system have been theoretically studied. In general, we find that the structure parameters of the coupled system significantly affect the optical susceptibilities. The enhancement of the coupling effects between the dot and ring is found to increase considerably the optical susceptibilities and redshift drastically the transition energies. Comparing to the linear susceptibility, the nonlinear optical susceptibility is found to be more sensitive to the variation of the structure parameters. A comprehensive analysis of the electron probability density movement with respect to the modification of the structure parameters is provided, which offers a unique perspective of the ground-state localization.     

Effects of extreme disorder on electronic properties in δ-doped semiconductor microstructures—a realistic computer simulation

Impurity induced disorder is a key feature of strongly doped semiconductor microstructures. We present a theoretical approach which allows the realistic and efficient calculation of localized quantum states in layered, delta-doped systems and the resulting properties of the quasi-2D multisubband electron/hole gas. The random Coulomb potential is directly computed from the impurity distribution without any simplifying assumptions. Electron-electron interaction is treated self-consistently on the Hartree level. The extreme cases of the doping superlattice (strong disorder) and the modulation doped quantum well (weak disorder) are studied as example device structures. Intersubband absorption spectra are then calculated for both types of systems and studied as a function of the electron filling factor. Striking differences are found between the linewidths of potential fluctuations and absorption spectra. These results are explained on the basis of system geometry, nonlinear screening and intersubband correlations. Finally, we discuss possible future applications and extensions of the method.     

Optical properties of an off-center hydrogenic impurity in a spherical quantum dot with Gaussian potential

Optical absorption coefficients and refractive index changes associated with intersubband transition of an off-center hydrogenic impurity in a spherical quantum dot (QD) with Gaussian confinement potential are theoretically investigated. Our results show that the optical absorption coefficients in a spherical QD are 2–3 orders of magnitude higher than those in quantum wells and are 2–3 orders smaller than those in a disk-like QD. It is found that the optical absorptions and the optical refractive index are strongly affected not only by the confinement barrier height, dot radius but also by the position of the impurity.     

Density of impurity states of shallow donors in a quantum well under intense laser field

The density of donor impurity states in a square GaAs–AlGaAs quantum well under an intense laser field is calculated taking into account the laser dressing effects on both the Coulomb potential and the confining potential. Using the effective-mass approximation within a variational scheme, the donor binding energy is obtained as a function of the laser dressing parameter, and the impurity position. Our results point out that a proper consideration of the density of impurity states may be of relevance in the interpretation of the optical phenomena related to shallow impurities in quantum wells, where the effects of an intense laser field compete with the quantum confinement.     

On intersubband absorption of radiation in delta-doped QWs

The results of calculation of intersubband absorption coefficients for either center-, or edge-delta-doped with Phosphorus 10 nm and 20 nm-wide Si_0.8Ge_0.2/Si/Si_0.8Ge_0.2 quantum wells are presented. It is shown, that the absorption for delta-doped structures differs substantially from that of a pure rectangular or uniformly doped ones. There are two main features for delta-doped quantum wells. The first one is the blue-shift for optical transitions between first and others (more pronounced), and second and others (less pronounced) space quantized energy levels. The second one is that edge doping changes the symmetry of the quantum well and forbidden optical transitions for the rectangular structure become now allowed. The influences of temperature, quantum well width, and impurity concentration on the optical absorption are studied. It is shown that the most dramatic changes in comparison with rectangular quantum wells are for wider investigated edge-doped structures with bigger number of ionized impurities.     

Hydrostatic pressure on optical absorption and refractive index changes of a shallow hydrogenic impurity in a GaAs/GaAlAs quantum wire

The effect of hydrostatic pressure on the binding energy of a hydrogenic impurity in a GaAs/GaAlAs quantum wire is discussed. Calculations have been performed using Bessel functions as an orthonormal basis within a single band effective mass approximation. Pressure induced photoionization cross section of the hydrogenic impurity is investigated. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of pressure are analysed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the pressure.     

Dynamic conductivity modified by impurity resonant states in doping three-dimensional Dirac semimetals

The impurity effect is studied in three-dimensional Dirac semimetals in the framework of a T-matrix method to consider the multiple scattering events of Dirac electrons off impurities. It has been found that a strong impurity potential can significantly restructure the energy dispersion and the density of states of Dirac electrons. An impurity-induced resonant state emerges and significantly modifies the pristine optical response. It is shown that the impurity state disturbs the common longitudinal optical conductivity by creating either an optical conductivity peak or double absorption jumps, depending on the relative position of the impurity band and the Fermi level. More importantly, these conductivity features appear in the forbidden region between the Drude and interband transition, completely or partially filling the Pauli block region of optical response. The underlying physics is that the appearance of resonance states as well as the broadening of the bands leads to a more complicated selection rule for the optical transitions, making it possible to excite new electron-hole pairs in the forbidden region. These features in optical conductivity provide valuable information to understand the impurity behaviors in 3D Dirac materials.     

电子-杂质和电子-声子相互作用对石墨烯磁光导的影响

在外加垂直磁场的石墨烯系统中,基于格林函数方法以自能的形式理论研究了电荷杂质散射和光学声子散射中心对朗道能谱的影响,采用久保(Kubo)公式研究了单层石墨烯的磁光电导谱以及跃迁选择定则。具体计算中电子-杂质库仑相互作用考虑了介电环境的屏蔽效应,对由散射引起的自能以及单粒子格林函数做自洽计算,另外在强磁场下单杂质散射是一个很好的近似模型。理论计算结果表明电荷杂质散射引起朗道能级对称展宽;同时考虑电荷杂质和光学声子两类散射后态密度表现为非对称的展宽。研究结果表明磁光电导谱的峰值和强度强烈依赖于填充因子和态密度。     

Kinetic theory of ionization and electron capture by a charged impurity in a semiconductor

Inelastic electron-phonon scattering in which the electron is captured or escapes from the Coulomb field of an impurity is taken into account in the kinetic equation for conduction electrons. This scattering is shown to become strong in a certain energy range. In this range, the distribution functions of free and bound electrons are correlated in such a way that there is a balance between the trapping and ionization processes. The existence of a region of strong scattering is the decisive factor in calculating the experimentally measurable trapping and ionization coefficients, which enter into the electron balance equation.     

Theoretical considerations regarding pulsed CO2 laser annealing of silicon

We present a calculation of the surface temperature and investigate the “thermal runaway” phenomenon during pulsed CO₂ laser (λ = 10.6 μm) annealing of silicon. In calculating the temperature variation of free carrier absorption in n-Si, we have taken into account acoustic deformation potential scattering, optical deformation potential scattering, and ionized impurity scattering. The deformation potentials are adjusted to fit the experimentally observed values at 300°K. Also, we discuss the contribution of free carrier absorption during annealing with a Nd:glass laser (λ = 1.06 μm).